Group B Streptococci (GBS) are the leading cause of neonatal sepsis and meningitis. Maternal IgG antibodies to the GBS polysaccharide (PS) protect the neonate from invasive GBS disease. Such antibodies are deficient in mothers of infants who develop GBS type III disease. A case control study was designed by NICHD to confirm the amounts of antibody needed for protection. As part of this study, GBS strains were collected from seven hospitals across the U. S. to study serotype prevalence. We developed a new inhibition ELISA for serotyping GBS strains, as the existing methods are not quantitative. For serotyping we used an intravenous hyperimmune immune globulin (IVIG) solution containing antibodies to GBS serotypes Ia, Ib, II and III, and a rabbit antisera against GBS type V. The specificity of the inhibition ELISA is comparable to conventional methods and inhibition ELISA is more sensitive and can also be used to quantitate capsular PSs. We have serotyped over 2000 strains from invasive cases and healthy controls. All the strains for the study have been serotyped and serotype Ia has been the predominant serotype (40%) among invasive cases. A case control study was done to determine the level of GBS type Ia associated with protection against early onset GBS disease. For each case we matched 4 heavily colonized, but healthy maternal/infant pairs. We determined the serum antibodies to GBS type Ia in the maternal and cord sera using the native GBS Ia PS in ELISA. A large percentage of both groups had less than 0.1 ug/mL of Ia antibody, but few maternal sera from cases had antibody levels over 5 ug/ml. The statistical analysis is continuing. Measurements of antibody concentrations for GBS type III have proven to be more challenging. GBS type III PS made by two different procedures as well as GBS type III PS conjugated to HSA when used in the measurement of antibodies, are all yielding different estimates. These results have been obtained by using PSs from commercial sources as well as other laboratories. We are now in the process of preparing GBS III PS from different procedures to study differences in binding to solid surfaces as well as probable modification of epitopes. We will use the standardized method to measure levels of protective maternal and cord serum antibodies to GBS type III.